Fuel mixing apparatus



Jan. 2, 1951 R. E. COE 2,536,678

FUEL MIXING APPARATUS Filed May 26. 1945 2 Sheets-Sheet 2 Envenfor Jan. 2, 1951 R. E. COE

FUEL MIXING APPARATUS .2 Sheets-Sheet 1 Filed May 26,- 1945 fil'tltafdl'. V W

stream iuel. mixer because bl operating upon the gases. the paired eat 99.31

Patented Jan. 2, 1951 FUEL MIXING APPARATUS Richard E. 'Coe, Salem, Ohio. assignor to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Application May 26, 1945, Serial No. 595,937

7 Claims.

The. invent o r ate t mix n a r and gas to form a fuel or gaseous mixture, and more particularly to a mixing apparatus for mixing gases downstream of a blower and. for maintainmg a constant or controlled ratio or relation ship of the gases, such as air and gas, through out an extremely wide flow range. Furthermore, thepresent invention constitutes improvements u n e u l M ng Apparatus of C pe Patent NO.- 2 341 7 The fuel miXil'lg apparatus of said Cope Patout No, 2,341,177 is satisfactory for maintaining a constant or controlled air-gas ratio over a wide ran e 01? flow; but it is an upstream lumen-that is. air and gas are mixed ahead of the b w rnd i subj t the di advan of all upstream mixers, in that the blower must hand e a c mbustible m t re w i h invol s a exp ion hazard nd is d u Prior downstream air-gas mixers use an oriflce in the air line, another orifice in the gas line, and a valve in series with one or the other orifice; the valve being; automatically controll d so as to attempt to maintain constantly related pressure differentials across the two orifices. However, serious difiiculties arise in such prior constructions because the pressure differential across an orifice varies as the square of the flow. With wide range of flow, there is an excessively high pressur or differential at one end of the range and an Q CQSSiVElY low pressure or differential at the other end of the range. Furtheh more, in such constructions, there is a transition from turbulent to laminar or streamline flow at low pressur differentials, accompanied by var.-

iation in orifice coefficients suflicient to introduce serious errors.

Moreover, in prior art fuel mixers, it is necessary to govern the gas pressure with a high degree of accuracy at low rates of flow; and available pressure governors cannot regulate gas pressures. under such Conditions with the reoui ed ree of precisi n- These difficulties have prevented prior downstream fuel mixers from maintaining a definite air eas rati th ou hout a Wide flow r g with the desi ed de re of accu y- In the upstream mix r OI the C91 8 Pa nt N zfitlrl'l'ih variations th pr ssure in the on duit for the mixed eases pstream of the blower are used. for ma ntaining a bst ntial y on-- stant pressure drop across the or fices. H w r. this x e ient. canno be in a do nel pressure is duit; and pressure variations in the mixed gas conduit do not bear the proper relation to-vol ume of flow to permit the control of orifices by this means.

In spite of the foregoing difiiculties, there is a demand for downstream fuel mixin apparatus in which the explosion hazard is absentee-which will maintain a constant selected air-gas ratio in a fuel mixture supplied to burners of various types of furnaces, irrespective of the rate of flow of the air-gas mixture to or through the burners, or in other words, irrespective of the burner fuel demand; particularly in connection with controlled atmosphere furnaces where large variations in demand are encountered.

I have discovered that the foregoing diflicule ties may be overcome and the problem solved in downstream fuel mixer construction that maintains a constant or controlled air-ga ratio Over Wide flow ranges, by providing for varying the relative rates of opening or closing of two valvesconstituting variable orifices-one in the gas line and another in the air line downstream of the blower; by governing the gas pressure in the gas line with relation to the pressure existing in the air line, upstream of their respective orifices; by maintaining the same or a constantly related pressure drop across both orifices; and by simultaneously controlling the valves to accommodate increased or decreased flow in response to a change in pressure drop across one of the orifices.

It is a primary obiect of the present inven-.- tion to provide new downstream fuel mixing apparatus which avoids the dangers and hazards of u stream fuel mixers.

Furthermore, it is an object of the present invention to provide downstream fuel mixing apparatus which maintains a definite, constant or controlled air-gas ratio throughout a wide flow range with the desired or necessary degree of accuracy.

Likewise, it is an object of the present invention to provide downstream fuel mixing apparatus which avoids prior art difiiculties arising from variations in pressure differentials across orifices incident to Variations in flow,

Also, it is an object of the present invention to provide new downstream fuel mixing apparatus in which the gas pressure may be governed in relation to the air pressure with the required degree of precision in a simple manner.

Moreover, it is an object of the present inven tion to provide fuel mixing apparatus in which a predetermined constant pressure drop is maintained across the air and gas orifices irrespective of changes in the size of said orifices or in the rate of flow therethrough.

Furthermore, it is an object of the present invention to automatically and simultaneously Vary the size of two orifices in fuel mixing apparatus to maintain the desired differential pressure across one of the orifices in response to a change in pressure drop across said orifice in order to accommodate increased or decreased flow.

Moreover, it is an object of the present invention to provide downstream fuel mixing apparatus for mixing air and gas in which the airgas ratio may be maintained within approximately 3% of a selected Value such as 10.5 to 1, 10.0 to 1, 9.5 to l, and 9.0 to 1 throughout a flow range of say or more to l.

Furthermore, it is an object of the present in-- vention to provide downstream fuel mixing apparatus having variable sized orifices in which adjustments may be made to compensate for variations in the flow coeificient of any orifice incident to a change in size thereof, or a change in rate of flow therethrough.

Furthermore, it is an object of the present invention to provide downstream fuel mixing ap paratus having a plurality of orifices adjustable as to size, which may be opened or closed simultaneously, with means for varying the rate of change of opening of one with respect to another.

Finally, it is an object of the present invention to provide new downstream fuel mixing apparatus which is simple in construction; durable, precise, effective and accurate in operation; and which overcomes the foregoing prior art difficulties and solves problems existing in the art.

These and other objects and advantages ap parent to those skilled in the artfrom the following description and claims may be obtained, the stated results achieved, and the described difficulties overcome, by the devices, constructions, arrangements, combinations, sub-combinations, parts, elements, discoverie and principles which comprise the present invention, the nature of which is set forth in the following general statement, a preferred embodiment of which-illustrative of the best mode in which applicant has contemplated applying the principleis set forth in the following description and shown in the drawings and which are particularly and distinctly pointed out and set forth in the appended claims forming part hereof.

The nature of the improvements in downstream fuel mixing apparatus of the present invention may be stated in general terms as preferably including a valve for one fluid such as air, a sec-- ond valve for another fiuid such as gas, said valves constituting variable orifices, means for varying the relative rates of opening or closing of the two valves, a blower upstream of said air valve, means for governing the gas pressure in the gas line with relation to the pressure existing in the air line upstream of said valves, means for maintaining the same pressure drop across both orifices by leading the fluid passing therethrough to a common mixing point, and means for operating both valves simultaneously to op or close the same to accommodate increased or decreased fiow actuated by a change in pressure drop across the orifice of one of said valves.

y y f example, a preferred embodiment of the improved fuel mixing apparatus is illustrated in the accompanying drawings forming part hereof wherein:

Figure 1 is a diagrammatic view of the improved fuel mixing apparatus;

Fig. 2 is a longitudinal section through one of the valves of the improved apparatus;

Fig. 3 is a section looking in the direction of the arrows 3-3, Fig. 2;

Fig. 4 is a longitudinal section of the other valve; and

Fig. 5 is a section taken on the line 35, Fig. 4.

Similar numerals refer to similar parts throughout the various figures of the drawings.

The improved downstream fuel mixing apparatus and its arrangement in a control system adapted for supplying a constant or controlled ratio air-gas mixture to one or more burners of a gas-fired furnace is illustrated somewhat diagrammatically in Figure 1.

The apparatus includes a blower 4 having an air filter 2 for supplying air under pressure to the air conduit 3 leading to the air valve generally indicated at 4%. Gas under pressure is supplied to supply conduit 5 and passes through a regulator 6 and through conduit '5 to gas valve generally indicated at 8. Pipes 8 and iii lead to mixing T H from air valve i, and pipes I2 and 13 lead from gas valve 8 to mixing T H. A conduit it leads from mixing T H to burner supply pipes l5. An explosion relief head 46 may be provided between air pipes 9 and I3; and a similar explosion relief head I! may be provided between gas pipes l2 and I3.

The air and gas valves and 8 are hereinafter described more in detail. Referring to Fig 1, they are both simultaneously operated by a rod !8 extending from valve actuating motor I 9. The operation of motor E9 is controlled by diaphragm controller generally indicated at 28 and by switches 2| and 22. The diaphragm 23 of controller 29 is spring loaded at 24, and one side 25 of the diaphragm chamber communicates through pipe 25 with the air conduit 3 immediately upstream of the air valve 4, while the other side 2'3 of the diaphragm chamber communicates through pipe 28 with the air pipe 9 immediately downstream of the valve 4 and upstream of the mixing T l l.

Gas pressure regulator 6 includes a diaphragm 29, an upper diaphragm chamber 30, a lower diaphragm chamber 3! and a valve 32 actuated by the diaphragm 23. The lower diaphragm chamber 3! communicates through connection 33 with the outlet side of valve 32, that is, with pipe 1. The upper diaphragm chamber 30 communicates through pipe 34 with air conduit 3. Diaphragm 29 is spring loaded at 35 and valve 32 may be spring loaded at 3B.

Proper operation of the downstream fuel mixing apparatus of the present invention requires that the gas pressure in gas line I be maintained or governed to have the same pressure, or a predetermined pressure differing by a constant amount, as the pressure which exists in the air line 3. This condition is maintained by the gas regulator 6 cross-connected to the air line 3 in the manner described.

Thus, the springs 35 and 36 are adjusted so that the regulator 6 maintains the same pressure in gas line I (or a pressure differing by a'constant pro-selected amount) as the pressure existing in air line 3. Whenever the pressure in gas line 1 falls below the pro-selected value, a reduced pressure occurs in chamber 3| and diaphragm 29 moves downward increasing the opening of valve 32, supplying increased pressure from supply pipe 5 until the desired relationship between 5; the measures iii. easoeiidoit-i the ahr oeiioeit 3 is. xeestahlisheci.

Gonvexsely, it. the easetessute ohiihit 1 heads to iheeease abo e t a desired. re ation be-- tweea it and the mea ure. at: onduit 3, die: nhiaem. 2% moves. upward, tending; to olose the valve- 32. and. .rediioe the as pressure item the supp y unt l the desired. condi ion is reestabl shed- Thus, gas regulator 6. by its. otoss oohheotioh with he airline itv gove ns the ga pressure in gas line i wit re ation. to e pressure exist a la he ah: line 3 at al times,

P1101261. operat n oi the presen iio he ieam. iuei mixing system also teq iires a oohetaht ol'fierent al. p essure or press re drop to he tained. at al times a ro s one oi the. valves a d as shown! th s ooht ol is maihtain eii. the ai alve 4-, With the amount of pressu atop de etmiheiii pring 2.4. (if tiiaphraem. 3. oi eoiiti'oilet Z0. is loaded so. that diaphragm Z n he eentral, or.- iieuti'al posi ion tor he. ete mined eonstaht pressure difietehtia i Wheh the pr s: sure difierential tends to increase, as. when the iuel dema d at the hrha e bothers increase diaphragm 2 moves down ard. and eve 3.1 091 ested therewith mo es. owhwarii to olose 1191: many ope miero s it h 42.:

n e eotri oir u t is hus establishe throu h litev 6 line. it, contacts All and ll o twitch 2 li e .2.f a1 e oohttoi m tor l3 and. line 43-. Valve actuating motor l9 operates to move rod 48 one diteo oh hi h begins to open alves 4 an 8- he openi oi. sate valves oontihues uhtil he se value o the pr ssure cho acr ss a r valve is reestablished when di ph agm ret rn to neutral position and ope ates lever 3.1 to open witch 2, thus sto pin motor I and stoppin th he o ening oi a es 4! and 8-.

Conv rsely, when the uel oeniaoii a the hi naee hiixhers deoreases, the p ess re d op or difier al valve ll tends to iiee e se thus .a chap agin 2t tow-are so that lever 31 oioses ihiorow oh ill When this e eetrie olrouit is estah lisheii through ne 3.8. oohtaots .4 and 5. of a switch 2|, line 46, valve control motor i!) and line 43. Valve actuating motor I9 operates, the ther iiireotioh to oloee alve and. .8 unt l h et va he oi the preesuie shop a oss valve l r e abl hed. wheh iiiaeh em 23 .ie hrns to n ii e i osnohi openin sw tch .2! and s op ing valvein t t, thu etop ihe thither olo ih: of a ves 4 ehtl ii.-

eihoe the. reset-ire eat l ahea of val e ehtrolleti by re or t wit irel tioh to the hi sshi'e a rline e aheasi oi ihe oth ii ahs t slisoharee into the eentii it ii wheteiii. or eeii e is he blowei th m asure .d op ao oss as .a e 8 maihtaiheti oohst n and i a fixed ela ion to the pressure iron aeross a r valve by the oehti l oi th PE1JIIQ d op ae oss a r valve ahii by the simulta eo s ooehins or oloshie of va ves't hil ii. to aoeommodate ohanees in now. Reieriihe to Figs i and 5 a r val e 4 may he a. .1. form of atii stahie tortv valve whioh a reotaheuiar ort 4.1 prov ded. in movable valve member 48, which 99i1 oi c oses a ross reote... iilar ports 4. in. a stationary valve m mei' tasi e. 5?..- The teotahehlar po t r ot f oe ti he a iustahle o si yhio ehient of member 5! the usual iiiahheh aha movable m er 4 be totatesl. t toiiehtruh ion 52 to whi h levee is. atta tii: he /er 3 tutti Thlifi;

is" eonneotee with too lt'oi ihotoe it.-

6 i movement. oi-

lt he the ehreetioh ta close valve 4,. nti moveiheht oi .ZTQQ i8 hi the oth eii eetioh to open. val e 4,

LBeo oi the iaet that the how 0 in ie oi va ying sized orifices change heo orifi es o valves 4 a d. .8. have diii e-nt sizes.- and. or other reasons, the theoretical r ation .e.stah lish ol and maintai ed by the oont oi of the he ditions described above cannot always a uaily p o u e r sults n exact aooo tiah e th the the retioal rel ions ip without pr viding e rtai d u m nts either one or hoth of valves 4 a d. 8.,

n the es nt iiie mixing appara s, the hi1:

' ditional adjustments are taken care of in the valve e ities. 2 and .3) w ioh may include a hous n .5 havin ectan u ar ports 5 and 5.6 ther n in which mo able valve in he 5! rotatahiy. mounted- Valve me ber 1 is pro: titled w th a rectangular port 511 and ec angu ar ort 59 which o en. or cl se actress reot he iar orts 5. and 5.6,. respect e Memhe 5 i ro ated for open n o o sihg t e valve. by rotation of valve stem 0 to whi h lever 6! is attaohei L er 5! is turn oehneoted to reel. .18 o valve o erat motor l.9 Movement oi. 1.8. in one d e tion end t olose valv 8 anti mo eme t of rod l8 the othe iireotion tones to. open v l e ":llhe efieetive o iiioe o valve aiid tion to gggnjng .ahol elosing by rotatio oi va e ste 5 may he a ju ted o en a ge or i'edue the size theieoi at any position o its mo em nt Th s is accgmpiished by pi tin lever 52. at. 53 on valve stem an linking its inner end .a 6 to alve member .57.. sprin 55 normally ur es m mber 51 down ard The outer end 55 oi l ver 55 eh: the u der surface o adjustable cam 61 p era ly f rmed. by a our ed sprin st el stri 58.. l athed up by ad hstihs s ews 53-.

Thu aojustab e oam surfaces. 61 may he. formed by a ustment oi scr ws '59 to aise or w r member 51. a any desired, po it on of IQ- tati ii o membe 51 to p ovide for ohaneing the rate of opening or closing of the effective orifice provided by valve 3, (in aoooriianoe with the results f tests on th s stem) in order to som pehsate fo changes n. the flow oo=eii oient o the. rifice or v rious s zes thereof, or io chan es n th flo therethroueh, or or oth r onhalahoes in. the s stem.-

.In the d awin s, val is indicat d. as air valve. and valve '8. is indicated as a gas valv but by m nor h nges in sizes and arransemehts of sa d v ves; alve 4 could. be he gas valve and val e could he the ir alv M-QEQOWL th se val es ha e been deso ibetl as, be ng ah an gas. va ves, hi t the s stem m y he used t t mixin ther g ses than ai ah fu l gashik iee, the cam a justment for ary ng th tats oi c ang of the ei eot e size oi one. of the orifices is iiidio t cl b ing n the gas valvehiitt is adju tmen e ovided i the ai val e, i d sired, H w r. if the am ad ihstmeht is. provided i the. a r al e. the cone heo ion of c ntrol pipes 2t and :28 for oohtroller 20 shoul he her es the orifice of the as valveot er ords, he oohtiol. or maintai i g a. con ant p essur tirop aeross one o the sho pr erabl be eonne teti. heroes the. whieh is no operated,

When the equi ment illustrated siiafe'ta. a is oally the l is ihstalleo for any ietaaoe the the air-gas ratio to be maigtainegi at a @591? 7', termined or constant rate over a large burner turn-down range is established, the valve settings of valve 8 are temporarily made and the equipment tested to determine fluctuations in the air-gas ratio. From the results of such tests, set screws 69 are adjusted to compensate for unpredictable changes in the flow coefiicients and for any other unbalances in the particular installation.

If at any time a different air-gas ratio is desired to be maintained, it may be obtained by adjusting the size of adjustable port 41 of valve 4 by movement of member 5|, and by then balancing the system by adjustment of valve 8 if necessary.

Alternatively and preferably, the overall airgas ratio may be adjusted by adjusting the spring loading of pressure regulator 6. When this method is used, settings may be determined by trial for various ratio requirements, and a previous setting readily duplicated at any time.

The improved fuel mixing apparatus of the present invention has been successfully used to satisfactorily maintain and supply a constant desired air-gas ratio within about 3% of a selected value over a 20 or more to 1 burner turn-down range. Normally, if the cam of valve 8 is adjusted to obtain a predetermined ratio such as a 10.5 to 1 air-gas ratio, the over-all ratio may be changed, by adjusting the curtain of valve 4, or by adjusting the spring loading of pressure regulator B, to some other air-gas ratio such as 10.0, 9.5 and 9.0 to 1 without disturbing the cam settings originally made on valve 8. In no case was there an observed deviation from the desired point greater than plus or minus 1.5% and in most cases the deviation was less than 1%.

V The cross-connected regulator 6 maintained thedesired pressure in the gas line I in an exceedingly accurate manner. At high flow, the air pressure may exceed the gas pressure by an amount equal to approximately .1" mercury column; but this deviation is not sufiicient to affectv the maintenance of the desired air-gas ratio.

In comparison to other methods of downstream mixing, the present system has a decided advantage of not depending upon orifice performance to provide impulses for the ratio controlling valve. Plain orifices are unreliable over wide ranges of flow because the critical point in flow characteristic does not occur at the same time in both gas and air orifices, with the result that impulses are imparted to the ratio controller which are not in accordance with the actual flow ratio.

The variable area orifices provided by the valves 4 and -8 eliminate this difiiculty because the pressure differential across the valves are maintained practically constant regardless of the rate of flow.

A further advantage of the system illustrated and described herein is the simplicity of the controller 2e which only uses a single diaphragm unit with a simple linkage connection with switches}! and 22. A controller of this type is not required to beoperative to control to exceedingly accurate limits because tests have shown that a 1" H2O deviation from the set point of constant pressure differential across the valve controlled has no measurable efiect upon the maintenance of the desired air-gas ratio. On the other hand double diaphragm controllers required in prior art arrangements actuated from fixedor'ifioes in gas and air lines require an ex tremely close control.

Accordingly, the present invention provides a downstream fuel mixer which avoids explosion hazards; which uses adjustable port gas and air valves as variable area orifices operating at 'a" predetermined constant pressure differential across the valves; which adjusts the valve positions automatically to maintain the desired differential pressure across one of the valves by differential pressure control and switch means receiving impulses from both sides of the valve and energizing a motor drive for the Valves; which governs the gas pressure in the gas line with relation to the pressure existing in the air line; which delivers a desired or predetermined air-gas ratio reliably over a wide range of flow; and which may be adjusted to change the predetermined air-gas ratio to be maintained.

Accordingly, the. fuel mixing apparatus of the present invention eliminates prior art difficulties, solves complicated problems which have existed in the art, and provides for accomplishing each of the foregoing objects in a simple and reliable arrangement.

In the foregoing descri tion, certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are utilized for descriptive purposes herein and not for the pur pose of limitation and are intended to be broadly construed.

Moreover, the embodiment of the improved construction illustrated and described is by way of example, and the scope of the present inven-v tion is not limited to the exact details of construction of the various parts.

Having now described the features of the invention, the construction, operation and use of a preferred embodiment of improved fuel mixing apparatus, and the advantageous, new and useful results obtainedthereby; the new and useful devices, constructions, arrangements, combinations, sub-combinations, parts and elements, and reasonable mechanical equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.

I claim:

1. In downstream fuel mixing apparatus, a plurality of valves discharging into a common conduit, each valve having an orifice, means for operating said valves to simultaneously enlarge or reduce the orifice openings thereof, a blower connected with one of the valves upstream thereof, means for supplying gaseous fluid under pressure to the other valve upstream thereof, means for maintaining constantly related pressures in the sources of supply tosaid valves, means automatically actuated in response to a change in differential pressure across one orifice for. controlling said valve operating means to maintain a predetermined diiferential pressure across one orifice, and means for varying the relative rates of opening or closing of said valve means throughout their ranges of actuation so as to compensate for changes in characteristics of the valve orifices and thereby maintain a constant ratio of the fluid flowing through thevalves and into the common conduit.

2. In downstream fuel mixing apparatus, a plurality of valves discharging into a common conduit, each valve having an orifice, means for operating said valves tosimultaneously enlarge or reduce the orifice openings thereof, a blower connected with one of the valves upstream thereof supplying gaseous fluid under pressure thereto, means automatically actuated in response to a change in differential pressure across one orifice for controlling said valve operating means to maintain a predetermined differential pressure across one orifice, means for supplying gaseous fluid under pressure to the other valve, means for governing the pressure of the gaseous fluid supplied to said other valve with relation to the pressure of the gaseous fluid supplied by the blower to said one valve, and means for varying the relative rates of opening or closing of said valve means throughout their ranges of actuation so as to compensate for changes in characteristics of the valve orifices and thereby maintain a constant ratio of the fluid flowing through the valves and into the common conduit.

3. In downstream fuel mixing apparatus for mixing air and gas to maintain a controlled airgas ratio over a wide flow range, air and gas valves providing variable area orifices to accommodate changes in flow, blower means supplying air under pressure to said air valve, means supplying gas under pressure to said gas valve, means governing the gas pressure of the gas supply with relation to the pressure of the air supply upstream of the respective valves, means automatically actuated in response to a change in differential pressure across the orifice of one valve for simultaneously opening or closing said valves to maintain a predetermined differential pressure across the orifice of said one valve, and means for varying the rate at which the other valve opens or closes at places in its range of actuation with respect to the rate at which said one. valve opens or closes to control a condition of predetermined air-gas ratio throughout the range of flow.

4. In downstream fuel mixing apparatus for mixing air and gas to maintain a controlled airgas ratio over a wide flow range, air and gas valves providing variable area orifices to accom modate changes in flow, blower means supplying air under pressure to said air valve, means supplying gas under pressure to said gas valve, means governing the gas pressure of the gas supply with relation to the pressure of the air supply upstream of the respective valves, means automatically actuated in response to a change in differential pressure across the orifice of one valve for simultaneously opening or closing said valves to maintain a predetermined difierential pressure across the orifice of said one valve, means for varying the relative rates of opening or closing of said valves throughout their ranges of actuation so as to compensate for changes in characteristics of the valve orifices and thereby maintain a constant ratio of the fluid flowing through the valves and into the common conduit, and means for adjusting the area of one of the orifices to change the air-gas ratio of the delivered mixture.

5. In downstream fuel mixing apparatus for mixing air and gas to maintain a controlled airgas ratio over a wide flow range, air and gas valves providing variable area orifices to accommodate changes in flow, blower means supplying air under pressure to said air valve, means supplying gas under pressure to said gas valve. means governing the gas pressure of the gas supply with relation to the pressure of the air supply upstream of the respective valves, means automatically actuated in response to a change in differential pressure across the orifice of one valve for simultaneously opening or closing said valves to maintain a predetermined differential pressure across the orifice of said one valve, means for varying the rate at which the other valve opens or closes at places in its range of actuation with respect to the rate at which said one valve opens or closes to control a condition of predetermined air-gas ratio throughout the range of flow, and means for adjusting the area of the orifice of said one valve to change the air-gas ratio of the delivered mixture.

6. In downstream fuel mixing apparatus for mixing air and gas to maintain a controlled airgas ratio over a wide flow range, air and gas valves providing variable area orifices to accommodate changes in flow, blower means supplying air under pressure to said air valve, means supplying gas under pressure to said gas valve, means governing the gas pressure of the gas supply with relation to the pressure of the air supply upstream of the respective valves, means automatically actuated in response to a change in differential pressure across the orifice of the air valve for simultaneously opening or closing said valves to maintain a predetermined differential pressure across the orifice of the air valve, and means for varying the rate at which the gas valve opens or closes at places in its range of actuation with respect to the rate at which the air valve opens or closes to control a condition of predetermined air-gas ratio throughout the range of flow.

7. In downstream fuel mixing apparatus for mixing air and gas to maintain a controlled airgas ratio over a wide flow range, air and gas valves providing variable area orifices to accommodate changes in flow, blower means supplying air under pressure to said air valve, means supplying gas under pressure to said gas valve, means governing the pressures of the gas and air supplies with relat on to each other u stream of the respective valves, means automatically actuated in response to a change in difierential pressure across the orifice of one valve for simultaneously opening or closing said valves to maintain a predetermined differential pressure across the orifice of said one valve, and means for varying the rate at which one of the valves opens 02' closes at places in its range of actuation with respect to the rate at which the other valve opens or closes to control a condition of predetermined air-gas ratio throughout the range of flow.

RICHARD E. COE.

REFERENCES CITED Theifollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,192,240 Schmidt Mar. 12, 1940 2 ,341 117? Cope Feb. 8, 1944 2,379,633 Garretson July 3, 1945 

